Electronic amplifier for motor controls



QRTI

R. R. CHAPMAN ELECTRONIC AMPLIFIER FOR MOTOR CONTROLS Filed May 9, 194e April 3, 1951 ya M M,

Patented Apr. 3, 1951 UNITED STATES PATENT OFICE Delaware Application May' 9, 1946, Serial No. 668,362

(Cl. 25W-2) .1. The: present invention is concernedwith an improved amplier of the type incorporating a Aphase discrimina-tor stage'.

It is often desirable in connection with condition control systems to provide an amplier which is capable of amplifying a signal voltage whichI var-ies in magnitude and phase with a given condition, the'amplier being' effective to supply toa motor orother'electrirca'l device oper'- ating a condition changing means, a voltage which is.A reversible` in .phase ini accordance with the signal voltage. Since the electrically operateddev-ice often op'eratesfin' amanner depend- .ent upon Vthe phase relationship 'between the applied to the discriminator stage, current' is supplied to' the electrical'device. A typical-ar'- rangement of this type isv that shown in the co-pending. application of Albert` P. UptonSerial No. 437,561yn1edApril 3,1942, now U..S.,Patent No. 2,423,534; In such an arrangement, it' is highly desirable lthat the signalvoltagebe amplined in the earlier' stagesof the amplifier without. any great distortion. Otherwise, the discriminator stage may operate erratically. In many condition responsive systems, however, there is a very great variation in the amplitude oft-he.V signal voltage and if the system isdesigned Yto be sensitive with very small vsignal voltages,

such as is' necessary if accurate control of the condition is maintained, it is `very dicult to provide for the amplification of large signal voltages without. excessive distortion.

An object of the present invention is-to-provide anarrangement in whichthe voltagesupplied to the input of the discriminator stage is maintained relatively constant'despite very large variations inthe input voltage.

A further object ofthe present-invention is toprovidesuch an arrangement in whichwlimiter -tubes arefprovided for limiting the maximum vamplitude of the voltagev which-can-besupplied to the input :circuit ofany one stage.-

'A still further obj-ect of thef presentr invention isto provide amultest-a-geamplifier havng a power stage supplied with1 an alternating source of power and at least one voltageamplification stage in which limiting means/'is providedv for lcausing the voltage ampliiication-stage tol supply a flat top input voltage to the input circuit ofthe power stage.

A still further object of the presentV invention is to provide Ya multi-'stage amplier having novel meansffor clipping a voltage amplified bythe ampliiier. Y

Other objects of theinvention will be-apparent from a considerationof the accompanying speciecation,-claims, and drawings; of whichl 'Ilhesingle figure shows schematically myimproved amplifier` employed inconnectionvvith-l a system for controlling the pressure Withingthe intake manifold :of an interna-l combustion engine.

vReferring to the figure-,my invention is-Shown therein, by'way of example-asapplied-tofa conftrol system whereinthe position of-a= valve, suc h as a Wasteg'atejotaturbo supercharge'r (not shown) is4 modulated4 in accordancel with` the balance or imbalance of abridge circuit I In the system, the unbalance potential of-th'e bridge circuit I l is-applied-'to the inputy of an-'aniplifier having two preliminary voltage amplication stages 1 -3 and IA; andan output discrimina-tor Vstage t5. A-motor IB o f the split phase type-has one phase supplied directly from asource of power,Y and the other phase supplied fromthe outputcircuitiot'theamplifier stage l5; Power is supplied to the system* from any suitable source of alternating current, to which isf corinected primary winding 2li ofn a transformen-2|. Transformer 2lhas a vplurality of secondary winding-sV 2'2, `23;v 2li,v and-25. Winding 22 supplies platevoltage tothe-outputstage l 5. Winding 23 supplies heater voltage for they tubes in theI ampliiier.V Wind-ings i 24 fand 2 5 1 are connected to rectiiierf tube- 2-6- of a conventional' f-ull wave rectifierv circuit.

Motor I6 isprovided-with 'apair of iieldwindings-2l and 28 anda'rotor 29.- As statedv above, the motor` is.- of thesplitrphase type` wherein,-a s is Wellr knownin the artyiieldwindings Eiland@ arey displaced in space phase,. and whereinthe displacement in-v time phase of the currentfiiow# ing. through the windings determinesA the direction of rotation-.of'-therotor 29. Rotor-E-Sd-rives aigear train generally indicated at 33. Theout put shaftv` of the gear train 30 isfconnected` to position the-waste gate l-UWthrougha-mechanical engine. *fthe intake manifold with the interior of a bellFlows 53.v A second bellows 54 is evacuated, so

linkage 3|. The output shaft of gear train 36 also drives a sliding contact 34 across a slide- Wire resistor 35.

The bridge circuit |I includes a pair of slidewire resistors 35 and 36, having a pair of slider arms 34 and 31. The bridge circuit has a pair of input terminals 38 and 39 and a pair of output terminals 46 and 4I. Output terminal 46 is the point of engagement of slider 34 andA slidewire resistor 35. Output terminal 4I is the point of engagement of slider 31 on slide-` wire resistor 36. Input terminals 38 and 39 are connected to the terminals of transformer secondary winding 42 of a transformer 43. Primary winding 44 of transformer 43 is connected to the same alternating current source as primary winding 26 of transformer 2|.

The bridge circuit II includesthe secondary winding 42, across whose terminals slidewire resistor 36 is connected; A slider 31 cooperates with resistor 36 and is connected to conductor 45, which is grounded'at junction point 46.

'Slider 31 and resistor 36 form a main controller "within a conduit which, for example, may be the vintake manifold of an internal combustion A pressure take-ofi duct 52 connects :that its expansion and contraction depend only upon atmospheric pressure. The two bellows 53 Vand. 54 are mountedwith their free ends extending toward eachother, so 'that the system is independent of atmospheric pressure. The

fcenter point on `a link 55 lbetween the free ends I'is connected, as by a pin and slot connection,

v'with the slider a1.

l vThe second slidewire resistor 35 is also connected across the terminals of Secondary wind- Slider V34 cooperates with the resistor 35- and is connected to a `conductor 56. Slider 5'34 with resistor 35 forms a follow-up potentiometer r51. Slider 34 is moved along the resistor ""35 bythe motorI |6, acting through the gear "train 36, and concurrently with the movement :fof the Waste gate I6.'

Output terminal 40 of bridge circuit is connected to a 'control grid 66 of a triode 6| 'through the slider 34,' conductor 56, resistor 62,

'and conductors 63 and 64. Output terminal 4| is connectedtofa cathode 65 of the triode 6I 4`through aislider 31, conductor 45, resistor 66,

and conductorl 61. The triode 6| in the first preliminary stage I3 also includes an anode 68 jtrol grid 12, 'anode 13, cathode 14, and heater 515. The control grid 1230i triode 1| is externally I connectedto theanode 13 at junction point 11 so that the triode functions as a diode. The cathode 14 of-triode-TI is connected to the control grid 660itriode 6I by conductor 64. Anode --13 of triode 1| is lvconnected lto the cathode 65 .of triode 6| by 'conductor 18, resistor 80, conductor 45, resistor 66, and vconductor 61. This y'eiectivelyconnects the triode 1| in parallel with 'the input signal to the-rst preliminary stage I3.

AThe re'ctier circuit- 85,- which is of the conventional -ful1fWave'type,'furnishes plate volt- Atage to the two preliminary stages I3 and I4, and bias voltages to al1 three stages of the amplier. The rectier tube 26 has a pair of anodes 86 and 481 which are connected to secondary winding 24 by conductors 88 and 89. The heater elements 96 and 9| are connected to secondary winding 25 by conductors 92 and 93. The rectifier tube 26, which may be of the type commercially known as type 6X5, also includes cathodes 94 and 95. The input of a iilter net- Wwork 96 is connected to the cathodes 94 and 95 of the rectier tube 26 by means of a conductor 91. The lilter network 96 includes a choke coil 98 and a pair of condensers 99 and |66. The positive output terminal |02 of the lter network 96 is connected to the grounded conductor 45 through bleeder resistors |63 and 66. Resistors 86, |64, and |65 are connected bej tween the grounded conductor 45 and the center tap |66 of the high voltage secondary winding 24. A positive unidirectional potential, with respect to the grounded conductor 45, is available at junction points |62 and |61. A negative unidirectional potential, with respect to the grounded conductor .45, is available at junction points |68, |69, and II6.

The input circuit of the rst preliminary stage I3 may be traced from the control grid 66 through conductors 64 and 63, resistor 62, conductor 56, slider 34, bridge circuit output terminal 46, bridge circuit bridge circuit output terminal 4 I, slider 31, grounded conductor 45, resistor 66, conductor 61, to the cathode 65 of triode 6|.

The output circuit of the first preliminary stage I3 may be traced from the positive output terminal |62 of the power supply through conductors |26 and |2I, load resistor |22, conductor |23, anode 68, cathode 65, conductor 61, and resistor 56 to the grounded conductor 45.

The output circuit of preliminary stage I3 is coupled to the input circuit of the second preliminary stage I4 by a condenser |24, one terminal of which is connected through conductor |23 to anode 63 of triode 5|, and the other termi-A nal of which is connected to a control grid |25 of a triode |26 by conductors |21 and |28. The condenser |24 serves to transmit any alternating signal appearing in the output circuit of stage I3 to the input circuit of stage I4, while blocking the plate voltage of stage I3 from the input circuit of stage I4.

The triode |26` of the second preliminary stage I4 also includes an anode |29, a cathode |36, and a heater I3I. Triode |26 forms one-half of the twin triode |32, which may -be of the type commercially known as type-12SL7. The twin triode |32 also includes a triode |33 including a control grid I34, anode |35, cathode |36, and heater |31. The control grid |34 of triode |33 lis externally connected to the anode |35 at junction point |33 so as to cause triode |33 to function as a diode. The cathode |36 of triode |33 is connected to the control grid |25y of triode |26 by a conductor |26. The anode I35 triode-|33 is connected to the cathode |36 of triode |26 by conductor |39, resistor |64, resistor 86, and conductors 45 and |43. This effectively connects the triode |33 in parallel with the input signal to the second preliminary1 stage i4.

The input circuit of the second preliminary stage I4 may be traced from the control grid |25 through conductors |28, |21, and |46, resistor |4I, conductors |42, 45, and |43, to the cathode |30 of triode |26. The output circuit of the second preliminary stage I4 may be traced from the positive output terminal |62 ofthe power supply' through conductors |1201- and |44', road re- S'istor |45, conductor' |46-, anode |26; cathode |30', and" conductor |43 to theA grounded conductor 45. rThe output stage l5 includes a pair oft-riodes |'U'and |5| in a twin triode |52. Triode lincludes an anode |53, a control grid |54, and a cathode |55. Triode llti includes an anode |56, a' c ontrol grid |51, and a cathode |58. Twin tri'o'de |52 also includes a suitable heaterv element |59` for heating both of the'cathodesl |555A and |58;

The control electrodes |54 andV |51 are connected tor a common junction |60 by means of conductors |6| and |62. The cathodes |55 and |58 are internally connected in twin triode I 52', which may be of the type commercially 1irnown as typeY SC3-3. The cathodes |55' and |58 are connected to the grounded junction point 46 on conductor 45 by means `of conductor |63. The" output circuit of the second preliminary stage I4 is coupled to the input circuit of the linal stage I5 by meansof a coupling condenser |64. One terminal of the condenser |64 is connested by conductor |46 to anode |29 of triode |126, the other terminal being connected to the junction point |60 of the control grids |54 and |-51 by conductor |65.

The input circuit of the nal output stage may be tracedv from the junction point |60 of the control grids |54 and |51 through conductor |65,

resistor |66, conductors |51 and |66., resistors Q |05, |04, and 80, to the grounded'conductor45.

Al filter condenser is connected between "junction point |1| of conductors |6| and |62 and the grounded conductor 45' by means of 'conductors |12 and |13. The condenser |10'by 'passes high frequency components appearing in the output of stage |14 and passing through the coupling condenser |64, thereby substantially preventing such components from affecting the control grids |54`and |51.

The output circuit of triode |50 may be traced from thefupper terminal of' secondary 22 through conductor |14, anode |53, cath-ode |55,

Vconductor |63, grounded junction points 46 and 11T, motor winding 28 and conductor I15vb'ack to thefcenter tap |19oi secondary 22.

The output circuitrof triode |5|l may betraced 'fromthe lower terminali of secondary 22 through 'conductor |18, anode |56, cathode |58, conductor 163;. grounded junction points46-and |11, winding=28 and conductor |15l back tothe center tap Motor winding 21 may be supplied with energy 4from a suitable alternating current sourceesuch junction point |80,- through conductor ISI andcondenser |82.

' Condenser |62- is chosen so as to form with winding 21 a series resonant circuit. Sincethis circuit is resonant. the current flowing init, and

hence the current in motor winding 21, isin time 'phase with the terminal'pot'ential'oftransformer Winding 26. The terminal potential' of; winding 21 leads this current by an yangle of approxi'- mately 90 electrical degrees, due to the inductance of the winding.

Motor winding 28-'may receive electrical energy in two ways: (l) By transmission from output stage |5 through conductor |15, and (2)' By i'nduction through transformer action from motor winding 21 when the motorris running. A condenser |16 is connected' across the terminals of motor winding v8. This condenser |16 is so chosen with relation to the impedanceofl winding 28 that the two together` forma parallel circuit resonant at the supplyA frequency. It will" be seen 'that condenser |16-` and' winding 2-6` fortn a parallel resonant circuit of high impedance withV respect to external current transmitted? to winding28-'through-conductor |15. On the other hand, condenser |16 and winding 28 form a series resonant circuit of low impedance with re spect to internal current induced in winding 28 by transformer action from winding 21.

The fluxes produced by each ofthe windings 21E and 20 in the motor do notV link the other winding when the motor is standing still. lWhen thermotor is rotating, however, the-armaturelresaction distorts the magnetic field in themotor so that winding 28 is linked' byV a portion ofthe iiuX produced by winding 21. This ux induces a current in winding 28, which is the current previously referred to as being produced by transformer actionV under runningl conditions.

The impedanceof` condenser |16 and winding V20 isfsuch that maximum power is-appliedto the motor with minimum anode dissipation in triodes |56 and |5| for the maximum allowable anode supply voltage.

The'values-set forth in the-following table are provided to illustrate more completely the spe'- cic-amplier circuit which has been constructed to carry out the principles of my invention. -It

should be understood, however, thatv these values Reference SuggestediVlues Numeral Characteristic 4 15,000 ohms. 470,000 ohms. l megohm;

220,000 ohms. oh'ms.

ohms;

25.000 ohms.

560 ohms. 5,000 ohms.

0.05 microfarad.

0.1 microfarad.

A0.01rnicrofarad.

0.001 microfarad.

. 395 volts each side ettari-179.

12.6 Volts;

360 volts each sideof tapl. 6.3 volts.

+1.l volts.

9.0 volts.

73.0 volts.

Resistance Operation` When the bridge circuit is balanced, the output terminals 40 and 4| are at the same potential, and hence no signal is applied to the input circuit of the amplifier. Therefore, no

'signal is produced in the output circuit of the second preliminary stage I4 and no signal isL applied to the inputcircuit of the output stage I5. Triodes |50 and |5I have a negative potential applied to their control grids |54 and |51 from junction point H0 in the 'power'supply This negative potential is of such a magnitude as to limit the anode currentof triodes lrand' |5| to a negligible value. Therefore, 'the waste gate |'0 maintained in the position which it hasV previously reached.

Let it be assumed that-'the pressure in the conduit 5| increases. This causes a slider arm 31 tomove to the left as it appears on the-drawing producing an unbalance of the bridge circuit in such a direction that 'the potential appearing between sliders 34 and 31 is ofthe same phasefajs thepotential between terminals 3B and 38, respectively. Let usarbitrarily'select triode |50'in the moutput discriminator stage l5 as the triode section of twin triode |52 which is supplied with plate potential in time phase with the unbalance potential from bridge circuit when the slider arm 31 responds to an increase in pressure in conduit 5|. The signal is transmitted through the two preliminary stages and appears on the input circuit of the output stage in phase with the output voltage of the triode |50, thereby rendering triode |50 conductive. The output stage l5 then supplies to the motor winding 28 an alternating potential which is in time phase with a potential in the upper half of transformer secondary winding 22, as it appears on the drawing. The current through winding 28 lags behind this potential due to the inductance of the winding.

with current in phase with the supply potential,

the two windings 2l and 28 are now energized with current displaced in phase, and rotation of -motor I6 follows. This rotation is transmitted through gear train 30 so as to drive waste gate l towards open position. The gear train 3|) also Vcauses the slider 3ft to move to the left along slidewire resistor 35, so as to restore the balance of the bridge circuit As soon as balance is restored, the signal to the amplifier again drops to zero, thereby stopping rotation of the motor I6.

In a correspondingy manner, a decrease of pressure in conduit 5| causes an unbalance of the bridge circuit H in such a direction that the phase of its output potential is shifted in the opposite direction. The potential applied to the input circuit of the output stage I5 is then in phase with the potential supplied to triode |5| from the lower half of transformer secondary winding 22, as it appears on the drawing. This potential causes triode |`5| to conduct. The current then supplied to theinotor winding 28 is opposite in phase to that supplied during the operation previously described. Motor IS therefore rotates in the opposite direction, driving the waste gate l towards its closed position and moving the slider 34 to the right along slidewire resistor 35, so as to restore the balance of the bridge circuit H'.

Where the signal voltage is applied by a bridge which is unbalanced according-cto the Value of a condition such as is shown in the present application, very large variations in signal Voltage may occur. For example, if the pressure suddenly changes by a very large amount, the amplitude of the signal voltage may be many times that occurring when the pressure change is very small or slow. If the pressure change is slow, in order to maintain the pressure accurately at the desired value, it is necessary for the amplifier to be capable of operating the motor when very small signal voltages occur. Normally, the pressure changes suiiiciently slowly so that upon theoccurrence of a very small pressurechange, the resultant movement of the waste gate tends to combat that pressure change and prevent it from ever exceeding a relatively smallvalue. Thus, any large sudden pressure change results in a signal voltage many times that at which the amplifier rst starts to cause operation of the motor. With a normal ampliiier of the type which have been used in prior art systems of this type, this tends to cause the signal voltage to be highly distorted in the voltage amplication stages so that in many cases, the voltage supplied to the grids of the discriminator is positive over more than half the cycle and has a negative peak of relatively short duration.A It will be apparent from y the foregoing description that where such occurs, the grids of the tubes in the discriminator stage will not only be positive during the half cycle in which the input voltage is in phase with the anode voltage of one of the tubes in the discriminator stage, but will also be positive during part of the other half cycle when the anode of the other tube is positive, so that both triodes `of the discriminator stage conduct though on opposite half cycles. Since they oppose each other, the power actually supplied to winding 28 is further decreased. By the use of my amplifier, I avoid these undesirable characteristics. To accomplish this, I have provided means for reducing the magnitude of both the positive and negative peaks of the input voltage whenever they exceed a predetermined value. With my arrangement, small signals of sinusoidal Wave form are amplied and applied to the control grids |54 and l5? of output stage I5 in their original form. As the amplitude of the signals increases, however, both extremities of the input wave form are limited to the desired level. This is accomplished by the limiting triodes 'H and |33, and bythe manner in which the triodes Si and |26 are connected into the circuit. This action will now be described.

Triode 1|, it will be recalled, is connected between the grid Si) and cathode 65 of triode 6| in series with resistor Sli of the power supply. Triode ll, acting as a parallel-diode limiter in the rst preliminary stage I3, conducts whenever the input reaches a more negative potential than the negative potential at junction point |08 in the power supply, or, in other words, that across resistor 8U. Current liows through triode 1|, acting as a diode, and series resistor S2. Since resistor 62 is large compared to the plate-cathode resistance of triode 1|, connected as a diode, essentially the entire input voltage is developed across resistor 62, limiting the negative half cycle to the value of the negative potential at junction point |08.

The grid-cathode circuit of triode 6| may be employed as a limiter circuit in exactly the same way as the plate-cathode circuit ofA triode 'l I, connected as a diode. The positive half cycle of the input voltage is limited to the Voltage level of the cathode 65, which is connected to a positive potential at junction point |07 in the power supply. During the positive portion of the input signal, the control grid 60 attempts to swing positive. Grid current flows through resistor 62 whenever the positive peak of the input voltage is greater than the voltage across resistor Et, developing a voltage drop of such polarity as to oppose the positive input signal. This limits the positive swing of the grid to essentially the cathode potential determined by the voltage across resistor 86, since resistor t2 is large compared to the gridcathode resistance when grid current is flowing.

It will be seen that as the input signal increases, the output wave form of the first preliminary stage I3 more closely approaches a rectangular wave. rlhis is an important point since the power output of the output stage |5 is dependent upon the wave form supplied to the control grids |54 and |51 of twin triode |52. l

The triode |33, acting as a parallel-diode-limiter in the second preliminary stage lll, limits the negative excursion of the controlling grid'vl25 of triode |26 to the value of the negative potential at junction point |09 in the power supply. It will be recalled that the resistors |134 and 80 connected between junction point |09 of the power supply and ground fare lconnected between the ,anode |35,and ground. Thus, current may flow through the vtriode |33 whenever the magnitude of the negative peak of the input voltage to triode I4' exceeds the voltage between junction point |09 and ground. Thus, the amount of the Anegative peak in the input circuit of triode |25 is automatically limited to the voltage between junction point |09 and ground.

The triode |26 acts to limit by its anode satu- 'ration characteristics the maximum value of the peaks of the output voltage during the half cycles vin which the grid ispositive. Thus, the swing of the output voltage in both positive and nega- 'tive directions is further limited in the second criminator stages approaches that of a rectangu- "lar voltage of fixed magnitude.

It will be seen that by the action of limiting triodes 7| and |33 and by the action of triodes t! and |25 in limiting the voltage, the input voltage supplied to the grids |54 and |51 of the discriminator stage is always of approximately the same magnitude regardless of how great is the signal voltage supplied to the input of the amplier by the bridge 1|. Thus, whenever the pressure change is above a predetermined relatively small value, full power is supplied to the motor l5 and this power remains substantially the same no matter how great the pressure change may be. Furthermore, the power supplied to the motor is much greater at even small signals than with prior amplifiers. This is due to the lack of undesirable distortion of the wave form. While the Wave form tends to approach that of a rectangular wave. this desirably aiects the operation of the discriminator stage |52. This discriminator stage has an alternating voltage applied to the anode, it will be recalled. By providing a ilat top input voltage to the discriminator stage, the sinusoidal wave form of the voltage applied to the output circuit is more nearly reproduced than where the input voltage to the grids is likewise sinusoidal. In other words, during the initial portion of each half cycle while the anode voltage is relatively small, the input voltage is at its maximum value which is assumed throughout the half cycle. Thus, more power is actually supplied to the winding of motor .28 than would be the case if the input voltage were likewise sinusoidal.

While have shown a specific embodiment of my invention for purposes of illustration, it is to be understood that the scope of the invention is to be limited only by the appended claims.

I claim as my invention:

1. An electronic amplifier comprisingr at least one voltage amplification stage and a discriminator stage, said voltage amplification stage comprising an electronic discharge device havnecting means `including a source of alternat- -ing voltage for applying lalternating voltage to each of the anodes of the devices of said discriminator stage, connecting' means for applying to each of the controlelements of said discriminator devices fa voltage which is determined by a voltage in said output circuit of said voltvage amplification stage, said connecting means said voltage amplification stage a variable magnitude alternating signal voltage reversible in phase and of vsuch phase relation with respect `to said source of alternating voltage that the voltages .applied to the control element and anode of one of said devices of said discriminator ystage tend to be in phase with each other and the voltages applied to the control element and anode of the other of said devices tend to ybe 18U out of phase with Veach other with the result that one or the kother of the devices is conductive dependent upon the phase of said signal voltage, and means for limiting the magnitude of the voltage applied to each of said control elements of said discriminator stage regardless of the magnitude of said signal voltage, said last named means comprising an impedance of relatively high value in series with the control element of said voltage amplification stage and a unidirectionally conductive device in parallel with said control element and cathode of said voltage amplication stage and being current conductive in a direction extending between said cathode and control element only when the negative peaks of the control element voltage eX- ceed a predetermined value.

2. An electronic amplier comprising at least one voltage amplification stage and a discriminator stage, said voltage amplification stage comprising an electronic discharge device having an anode, a cathode, and a control element, said discriminator stage comprising a plurality of electronic discharge devices each comprising an anode, a cathode, and a control element, means for connecting the anode and cathode of said device of said voltage amplification stage in an output circuit with a voltage source, connecting means including a source of alternating voltage for applying alternating voltage to each of the anodes of the devices of said discriminator stage, connecting means for applying to each of the control elements of said discriminator devices a voltage which is determined by a voltage in said output circuit of said voltage amplication stage, said connecting means being so effective to apply said voltages that the phase relationship between the voltages applied to the anode and control element of one of said devices of said discriminator stage is diiierent from that between the voltages applied to the anode and control element of the other of said devices of said discriminator stage, means for applying to the control element of said Voltage ampliiication stage a variable magnitude alternating signal voltage reversible in phase and of such phase relation with respect to said source of alternating voltage that the voltages applied to the control element and anode of one of said devices of said discriminator stage tend to be said unidirectional Ain phase With each other andthe voltages applied to the control element and anode of the other of said devices tend to be 180 out of phase with each other with the result that one or the -other of the devices is conductive dependent upon the phase of said signal voltage, and `meansfor limiting the magnitude of the voltage applied to each of said control elements of said discriminator stage regardless of the magnitude of said signal voltage, said last named `means comprising an impedance of relatively `high value in series with the control element of `said voltage amplification stage and a unidirectionally conductive device and a source of unidirectional voltage in series with each other and in parallel with said control element and cathode of said voltage amplication stage, the

.voltage necessary to cause conduction of said device being of such polarity as to` be in opposition to the voltage across said unidirectional source, and said device being current conductive in a direction extending between said cath- -ode and control element only when the negative peaks of the control element voltage exceed a predetermined value exceeding that of voltage.

ROBERT R. CHAPMAN.

12 REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,426,826 Egerton Aug. 22, 1922 1,788,401 VMeagher Jan. 13, 1931 1,977,624 Davis Oct. 23, 1934 1,989,829 Specht Feb. 5, 1935 2,115,881 Roosenstein May 3, 1938 2,176,663 Browne et al. Oct. 17, 1939 `,2,226,459 Bingley Dec. 24, 1940 2,322,218 Baird June 22, 1943 2,370,692 Shepherd Mar. 6, 1945 2,406,856 Satterlee Sept. 3, 1946 2,407,876 Godet Sept. 17, 1946 2,425,733 Gille et al. Aug. 19, 1947 2,426,497 Field Aug. 26, 1947 V2,426,711 Shaffer Sept. 2, 1947 2,446,563 Upton Aug. 10, 1948 2,450,479 Lindsay Oct. 5, 1948 FOREIGN PATENTS Number Country Date 552,118 Great Britain Mar. 24, 1943 

